Slide dupe film for Kodachome was a specialist thing and was discontinued long ago.

Fuji CDU is good for e-6 transparencies, and it still turns up on the auction site from time to time.
Be prepared to tweak exposure, gelatin cc filtration fine tuning, and do H-D plots of 3 colours of a step wedge/ grey patch target read through a denistometer to nail the exposure and filtration with this process, once you have found an e-6 processor or your own efforts that deliver consitent development results.

Very rewarding when you pull all of the required stars into alignment, but not a task that is taken lightly.

I usually build a list of needed dupes and gear up the process to get it done right once every two or three years, as a batch effort in my 35mm slide duper, or polaroid process camera using an enlarger head inverted as the adjustable light source, and filter it to daylight with filters in the head, and tweak film response with CC filters in the lens path. That lets me go between the tow camer rigs and keep the flash/daylight ambient metering working.

In between, a roll of 400 or so feet of CDU that I snip from lives in the back of the freezer, with updated exposure and filtration from each run written on the can with a sherpie so I can try to spont trends to cut the testing cycle the next time I pull it out.